1. Field of the Invention
This application relates to a header for mounting a planar transformer on a motherboard, and in particular relates to a header for providing electrical isolation and mechanical fixture of the planar transformer.
2. Description of the Related Art
Planar transformers are commonly used in power supply units, and are often used on motherboards or main printed circuit boards (PCBs) in preference to other types of transformers when a low design profile is required. In these cases, the windings of the transformer can be incorporated into the main PCB, although this is at the expense of main PCB board area that could otherwise be used to mount other components related to the power supply circuitry.
As an alternative to incorporating the transformer windings directly into the main PCB, the transformer can instead be mounted above the PCB. This necessarily increases the design profile of the board because of the greater extent to which the planar transformer protrudes from it, but has the advantage that the footprint area that the transformer makes on the board can be reduced. This increases the amount of space on the board that can be used for mounting other components.
Whether fitting a transformer above a main PCB (or motherboard) or directly incorporating the windings into the main PCB, it is known that the transformer must be insulated from the components on the PCB to which it is mounted. This is especially important if the transformer forms part of a high-voltage isolated circuit. In this situation, industry standards, such as the EN/UL60950 standards guide, require that certain minimum clearance and creepage distances are maintained. Thus, the shortest distance between two conductive paths on the transformer and the PCB, as measured through air (also known as the clearance) must be maintained above the industry standard value. Similarly, the shortest distance between two conductive paths on the transformer and the PCB, as measured along the surface of the insulation (also known as the creepage) must also be maintained above the industry standard value.
It is known in the art to provide a thin insulator or “shim” between the transformer and the main PCB upon which it is mounted. Subject to the shim having a certain minimum thickness, the requirements regarding clearance and creepage can be met using this technique. However, the use of a shim is often not ideal as it provides poor structural support to the transformer, and makes the manufacture of the circuit boards difficult, especially if automated soldering techniques such as reflow soldering are used.
It is also known in the art to completely encapsulate and seal the transformer in an epoxy resin or similar material before mounting it to the main PCB. However, such a technique is again unsuitable for reflow soldering techniques due to migration of solder into the epoxy resin, and the possibility of voids forming in the epoxy which, during reflow soldering, can expand and lead to a failure of the device.
It is desirable to provide an improved mounting of a planar transformer on a motherboard.